Brush seals

ABSTRACT

According to the present invention there is provided an annular brush seal comprising a bristle pack, the bristle pack having an upstream face, a downstream face and a sealing face, wherein the sealing face is provided with an annular chamfered relief extending from a location intermediate the upstream face and the downstream face and terminating at the upstream face. The revised geometry providing more consistent performance through the life of the brush seal. Also provided is a method for the manufacture of such a seal.

[0001] The present invention relates to brush seals of the kind used for providing a seal between relatively rotatable components. A typical application is in gas turbine engines where they are used to provide a seal between rotating and static components in order to control secondary airflow within the engine.

[0002] A brush seal between relatively rotatable first and second coaxial structures comprises an annular array of bristles called the bristle pack, held by a carrier arranged so that a continuous annular periphery of the bristle pack is exposed. The exposed annular periphery, henceforth known as the sealing face, engages the second structure which is able to move relative to the sealing face.

[0003] The invention concerns brush seals used to separate regions of a gas at different pressures. Henceforth, the region at higher pressure will be called the upstream region while the region at lower pressure will be called the downstream region.

[0004] In operation, the pressure differential across the brush seal forces the bristles at the sealing face against the second structure, an effect known as “blowdown” which effects particularly those bristles closest to the upstream region. Whilst producing a very good seal, blowdown leads to rapid degradation of the seal's performance. This rapid degradation in performance is caused by each or any of the following: wear, excessive and permanent bending of the bristles, and joining of the bristles in a solid mass, at least at the sealing face.

[0005] As a result, the performance of the brush seal is characterised by a marked initial degradation in performance. This slows to an acceptable level as the seal reaches a “plateau phase” when the bristles have worn to a length that sufficiently obviates the effects of blowdown.

[0006] Hence, while the performance of the brush seal in its plateau phase may be acceptable for an application, the disparity between initial performance and that at the plateau phase precludes the use of brush seals where they might otherwise be suitable.

[0007] The present invention seeks to provide an improved brush seal structure. In particular the present invention seeks to provide an improved brush seal structure that provides a consistent performance throughout the life of the seal.

[0008] According to the present invention there is provided an annular brush seal comprising a bristle pack, the bristle pack having an upstream face, a downstream face and a sealing face, wherein there is formed between the upstream face and the sealing face an annular relief in the form of a chamfer that extends from a location on the upstream face to a location on the sealing face intermediate the upstream face and the downstream face.

[0009] According to another aspect of the present invention there is provided a method for producing an annular brush seal comprising a bristle pack, the bristle pack having an upstream face, a downstream face and a sealing face, wherein there is formed between the upstream face and the sealing face an annular relief in the form of a chamfer that extends from a location on the upstream face to a location on the sealing face intermediate the upstream face and the downstream face.

[0010] The invention will now be described, by way of example and with respect to the accompanying drawings, in which:

[0011]FIG. 1 is a diagrammatic cross section through an assembly utilising a conventional brush seal between static and rotating components to isolate regions of gas at different pressure;

[0012]FIG. 2 is a cross section of a conventional brush seal;

[0013]FIG. 3 is a cross sectional view on line 1-1 through the brush seal of FIG. 2;

[0014]FIG. 4 is a cross sectional view of a brush seal according to the present invention in the same plane as FIG. 2;

[0015]FIG. 5a shows a cross section of the brush seal 21 prior to creation of the chamfered relief;

[0016]FIG. 5b shows a cross-section of the grinding wheel used to create the chamfered relief and;

[0017]FIG. 5c shows the application of the chamfered relief.

[0018]FIG. 1 shows a diagrammatic cross section through a static first structure 2 and a coaxial, relatively rotatable second structure 4. A conventional brush seal 6 fixed to the first structure 2 abuts the rotating structure to provide a seal between an upstream region 8 of gas at higher pressure than that in a downstream region 10. It will be understood that the brush seal 6 does not aim to provide a perfect seal between the upstream region and downstream region but instead seeks to reduce airflow from between the upstream and downstream region to an acceptable level.

[0019] Referring now to FIG. 2, a cross section of the brush seal 6 is shown in more detail. The brush seal 6 comprises a layered annular array of bristles called the bristle pack 12, the outer peripheries of which are trapped between an upstream ring 14, and a downstream ring 16. The proportions of ring 14 are such as to only cover, nominally, the radial outer 25% of the bristle pack 12. The proportions of ring 16 cover, nominally, 70% of the radial outer portion of the bristle pack 12. In an operating environment, the upstream ring 14 will face an airflow which results in 75% of the lengths of the leading layers of the bristle pack 12 being subjected to the upstream air pressure. The inner annulus of the bristle pack 12, called the sealing face 18, engages the second structure 4 about its periphery, providing a low-leakage seal.

[0020] Referring now to FIG. 3, the brush seal 6 of FIG. 2 is shown on line 1-1 through the brush seal of FIG. 2. Bristles within the bristle pack 12 are arranged so that when the brush seal 6 is supported in the fixed structure 2 about the second structure 4, the bristles engage the second structure 4 at a “lay-angle” 20 to it, which lay-angle 20 is non radial. During operation, the radial inner ends of the leading layers of bristles in the bristle pack 12 will be subjected to a force known as “blow down”. The air pressure differential between high pressure region 8 and low pressure region 10 maintained by the brush seal 6 forces the free end portions onto the surface of the second component 4, resulting in an excessive area of contact and hence wear. Initially the sealing performance of the brush seal 6 is very good, however the excessive wear caused by “blowdown” reduces the sealing efficiency of the brush seal 6 to an unacceptable level, allowing increased leakage of air from the upstream region 8 to the downstream region 10. Further, the force exerted by the pressure differential may cause the worn bristles to compact together and so become inflexible, which will further reduce the ability of the brush seal 6 to prevent leakage.

[0021] Referring to FIG. 4, a cross section through a brush seal 21 according to the present invention is shown. The construction of the improved brush seal 21 is substantially the same as the prior art and like parts carry like reference numbers. However, in order that the disadvantages described hereinbefore may be obviated, the bristle pack 12 is chamfered, indicated by the numeral 22. The chamfer 22 is annular, forming a conic relief in the bristle pack 12, extending from a point 24 intermediate the upstream face and downstream face and terminating at the upstream face. The chamfer 22 removes those ends of bristles that would otherwise experience “blow down”. As a result, any airflow across the brush seal collides with the brush seal 6 and turns radially inwards, losing momentum as it does so. The chamfer 22 smoothes the airflow onto its original course, to be all but stopped by the downstream layers of bristles, the resulting small leakage through which is acceptable. A further advantage gained by chamfering bristles as described hereinbefore, is the avoidance of compaction through obviation of “blow down”.

[0022] FIGS. 5(a-c) illustrate the process used to create the chamfered relief 22 described above.

[0023] Initially the brush seal 21 has an internal bore 26 smaller than the final internal bore 28 of the finished bristle pack 12. A grinding wheel 30 has a cylindrical first grinding face 32 of larger diameter than the required finished bore of the brush seal. A conical second grinding face 34 has an oversize profile relative to the finished chamfer 22 of the brush seal.

[0024] Referring now to FIG. 5c, in a final machining operation, the grinding wheel 30 is rotated and advanced into the undersize bore 26 of the unfinished brush seal, compressing the bristle pack 12. The first grinding face 32 of the wheel 30 dresses the bore of the bristle pack 12 to a diameter slightly larger than that required of the finished brush seal 21. Then, as the grinding wheel 30 is further advanced into the bore of the bristle pack, the second grinding face 34 applies the chamfered relief. As before, the relief created is oversize when compared with the final relief 22.

[0025] When the chamfered relief has been completed, the grinding wheel 30 is withdrawn from the bristle pack 12. Because the grinding wheel 30 no longer compresses the bristle pack 12, the pack 12 expands and so the bore 28 and chamfered relief 22 adopt the final dimensions required.

[0026] In a preferred embodiment of the invention, the chamfered relief applied 22 to the brush seal 21 is coaxial with and at substantially constant angle 32 to the sealing face 18. In a further preferred embodiment, this angle 32 lies between forty and fifty degrees.

[0027] Although the brush seal described above provides sealing about an inner sealing surface 18, it will be understood that the invention could equally well be applied to annular brush seals which provide a seal about an external diameter.

[0028] Although it is efficacious to apply a final dressing to the bore at the same time as the application of the chamfered relief, it will be understood that the two processes could be separated into different steps. Also, the use of a grinding wheel 30 of substantially similar profile to the internal dimensions of the bristle pack 12 may be replaced by other machining techniques such as orbital grinding utilising an undersized dressing tool or by use of a multi-axis machining centre. 

1 An annular brush seal comprising a bristle pack, the bristle pack having an upstream face, a downstream face and a sealing face, wherein there is formed between the upstream face and the sealing face an annular relief in the form of a chamfer that extends from a location on the upstream face to a location on the sealing face intermediate the upstream face and the downstream face. 2 An annular brush seal as claimed in claim 1, wherein the annular relief is formed on the bristle pack coaxial with, and at a substantially constant angle to the sealing face. 3 An annular brush seal as claimed in claim 2 wherein the angle of the chamfer to the sealing face lies between 40° and 50°. 4 A method for producing an annular brush seal comprising a bristle pack, the bristle pack having an upstream face, a downstream face and a sealing face, wherein there is formed between the upstream face and the sealing face an annular relief in the form of a chamfer that extends from a location on the upstream face to a location on the sealing face intermediate the upstream face and the downstream face. 5 A method as claimed in claim 4 comprising the further steps of using grinding means to form the annular relief in the bristle pack. 6 A method as claimed in claim 5 wherein the grinding means comprises a grinding wheel with a conical grinding face. 7 A method as claimed in claim 6 wherein the grinding means comprises a grinding wheel with a first cylindrical grinding face and a second conical grinding face. 